Presently, hybrid powertrains are an increasingly popular approach to improving the fuel utilization of motor vehicles. “Hybrid” refers to the combination of a conventional internal combustion engine with an energy storage system, which typically serves the functions of receiving and storing excess energy produced by the engine and energy recovered from braking events, and redelivering this energy to supplement the engine when necessary. This decouples the production and consumption of power, thereby allowing the internal combustion engine to operate more efficiently, while making sure that enough power is available to meet load demands.
Several forms of energy storage are known in the art, with electrical storage using batteries being the best known. Recently, hydraulic hybrids have been demonstrated to offer better efficiency, greater power density, lower cost, and longer service life than electric hybrids. A hydraulic power system takes the form of one or more hydraulic accumulators for energy storage and one or more hydraulic pumps, motors, or pump/motors for power transmission. Hydraulic accumulators operate on the principle of storing energy by compressing a gas. An accumulator's pressure vessel contains a charge of gas, typically nitrogen, which becomes compressed as a hydraulic pump pumps liquid into the vessel. The liquid thereby becomes pressurized and when released may be used to drive a hydraulic motor. A hydraulic accumulator thus utilizes two distinct working media, one a compressible gas and the other a relatively incompressible liquid. Throughout this document, the term “gas” shall refer to the gaseous medium and the term “fluid” shall refer to the liquid working medium, as is customary in the art.
In the present state of the art, there are three basic configurations for hydraulic accumulators: spring type, bladder type and piston type. Spring type are typically limited to accumulators with small fluid volumes due to the size, cost, mass, and spring rates of the springs. Bladder accumulators typically suffer from high gas permeation rates and poor reliability. Of these, the piston type is the least costly design that can store desirable volumes of fluid. In addition, properly designed piston accumulators are physically robust, efficient, and reliable.
Standard piston accumulators are also well represented in the art. In a standard piston accumulator, the hydraulic fluid is separated from the compressed gas by means of a piston, which seals against the inner walls of a cylindrical pressure vessel and is free to move longitudinally as fluid enters and leaves and the gas compresses and expands. Because the piston does not need to be flexible, it may be made of a gas impermeable material such as steel. However, the interface between the piston and the inner wall of the cylinder must be controlled tightly to ensure a good seal, and the degree of dimensional tolerance necessary to ensure a good seal may increase the cost of manufacturing. It also requires that the pressure vessel be extremely rigid and resistant to expansion near its center when pressurized, which would otherwise defeat the seal by widening the distance between the piston and cylinder wall. This has eliminated the consideration of composite materials for high pressure piston accumulator vessels, as composite materials tend to expand significantly under pressure (e.g., about 1/10 of an inch diametrically for a 12 inch diameter vessel at 5,000 psi pressure).
As a result of the foregoing, standard piston accumulator vessels tend to be made of thick, high strength steel and are very heavy. Standard piston accumulators have a much higher weight to energy storage ratio than either steel or composite bladder accumulators, which makes them undesirable for mobile vehicular applications (as such increased weight would, for example, reduce fuel economy for the vehicle). More specifically, piston accumulators for the same capacity (i.e., size) and pressure rating are many times heavier (e.g., by up to 10 times) than an accumulator with a lightweight composite pressure vessel design, as would be preferred in such applications where accumulator weight is an issue. Therefore, despite their potentially superior gas impermeability, piston accumulators are largely impractical for vehicular applications.
Several prior art piston accumulator concepts utilize a piston-in-sleeve accumulator design, in which the piston resides within and seals against a cylindrical sleeve that is separate from the inner wall of the pressure vessel. The sleeve is defined as a hollow member substantially incapable of withstanding stresses applied thereto were a full pressure differential of the accumulator applied across a hollow member. While this approach provides at least two benefits over the prior art: (i) separating the pressure containment function of the vessel wall from its piston sealing function, allowing an effective seal to be pursued with a sleeve independently of issues relating to pressure vessel construction, and (ii) providing an intervening or interstitial volume between the sleeve and vessel wall which may be filled with the charge gas to allow tailoring of the ratio of gas to fluid to optimize performance and which also allows shaping the pressure profile of the discharged oil.
A disadvantage of these systems is that such designs comprise a generally thick-walled strong cylindrical pressure vessel constructed of a steel alloy, and a metal sleeve that is thin relative to the vessel walls. The sleeve is permanently attached to the inner surface of one end of the pressure vessel near its circumference, creating (with the piston) a closed or “inside” chamber for the working fluid. The other end of the sleeve extends toward the other end of the vessel and is generally left open to create an “outside” chamber that consists of the open volume of the sleeve, the remaining volume of the pressure vessel, and the intervening/interstitial space between the outer wall of the sleeve and the inner wall of the pressure vessel, each filled with the gaseous medium of the accumulator.
Another disadvantage of these systems is the operation of such requires the sleeve to be tightly retained and centered within the vessel to prevent radial movement, for example, due to vibrations in use with mobile (e.g. aircraft) applications. Sleeve movement fatigues the rigid fixed end of the sleeve possibly leading to leakage due to cracking, distortion, or wear of the sealing gasket if one is present. This requires the sleeve to either be stiffened by connecting it at points to the vessel wall, or requires the sleeve to be thicker than the minimum that would be necessary to withstand the small pressure differentials normally encountered in charging and discharging. Further, the outer walls of the vessel must be thicker than would be necessary for pressure containment alone because the walls must be prevented from expanding and thus loosening the sleeve or distorting it from the true circular form necessary for piston sealing.
Prior art piston-in-sleeve designs also uniformly contain the fluid within the closed (inside) chamber, with the charge gas residing on the other side of the piston and in the interstitial space between the sleeve and vessel wall. This arrangement is naturally preferable because it maximizes the fluid capacity and hence energy capacity of the device. That is, the working medium that resides inside the sleeve may be discharged completely, while some portion of the medium outside the sleeve will always remain trapped in the interstitial space; because working capacity is determined by how much fluid may be discharged, it is a natural choice to have the fluid reside on the inside of the sleeve and gas on the outside.
Like standard piston accumulators discussed above, these prior art piston-in-sleeve accumulators are unacceptably heavy for a hydraulic hybrid motor vehicle application or other application where accumulator weight is a significant issue. Attempts have been made to reduce the weight of such piston-in-sleeve accumulators through the use of lightweight composite materials in place of steel for the pressure containment function in the vessel wall. However, such devices still require an internal metallic core to the vessel wall and a thickened metal area at one end of the accumulator. As such, the device remains undesirably heavy for a hydraulic hybrid motor vehicle application. The intense duty cycle experienced by the accumulator (i.e., the extremely large number of charge-discharge cycles, in some cases exceeding one million cycles) and the significant radial expansion of composite materials (about 1/10 of one inch diametrically for a 12 inch diameter vessel at 5,000 psi pressure) together would result in expected fatigue failure of the metal core or liner.
To resolve these shortcomings, prior art devices employ a thermoplastic sleeve, with a carbon-fiber wound pressure vessel shell. The device also places the fluid outside the core, thus the fluid fills the interstitial spaces. There are several significant issues with this design: (i) the physical size of the accumulator is larger than necessary to enclose the same volume of useful working fluid as the fluid in the interstitial spaces cannot be used; (ii) optimal accumulator design require that the gas volume be greater than the fluid volume; (iii) the design cannot be serviced—any failure of any component requires that the entire cylinder be discarded, (iv) the thickness of the pressure vessel wrapping is thicker than needed because the wrapping must counter both axial and tangential loads, and (v) the design does not provide the means to protect the integrity of the sleeve should the oil pressure exceed that of the gas pressure.
Most recently, a compact hydraulic accumulator has been developed that provides a serviceable piston and sleeve design using an extremely light-weight composite pressure vessel. The modular design provides accumulator cylinders and auxiliary gas cylinders in fluid communication via manifolds doubling as removable end caps with ties rods maintaining the module in tension. Such a device is disclosed in commonly owned U.S. Pat. No. 7,661,442, incorporated herein by reference in its entirety.
A drawback to this device is the bulk of the end caps housing the manifold along with the tie rods required to seal the vessel. These components add substantially to the package space required to fit into a vehicle. A more compact end cap would improve the utility of the accumulator by virtue of reducing its size and package requirements.